1. Field of the Invention
The present invention relates to a method and a device for signal decision, a receiver and a channel condition estimating method for a coding communication system. More Particularly, the present invention relates to a method and a device for signal decision, a receiver and a channel condition estimating method for a coding communication system which are feasible for mobile communication using, e.g., a digital handy phone or mobile phone.
2. Description of the Background Art
Recently, due to the spreading mobile communication environment, digital handy phones and mobile phones promoting the efficient use of limited frequencies have been standardized in various countries. In the USA, for example, a North American TDMA (Time Division Multiple Access) handy phone system (IS-54) and other TDMA handy phone systems have been standardized first, and then followed by a North American CDMA (Code Division Multiple Access) digital cellular system (IS-95) and other CDMA handy phone systems.
In the North American TDMA digital cellular system, a signal lying in the speech band is transformed to a code by a voice coding method called VSELP (Vector Sum Excited Linear Prediction). The code is converted to an error correcting code by, e.g., a convolutional code, CRC (Cyclic Redundancy Check) code, and interslot interleave. A synchronizing signal and control signals are added to the error correcting code in the format of a TDMA slot. Particularly, a speech signal and an FACCH (Fast Associated Control Channel) signal included in the control signals and used to switch cells are each encoded by a convolutional encoder having a particular code rate. For example, speech data and FACCH are encoded by a rate 1/2 and a rate 1/4 convolutional encoder, respectively. The resulting convolutional codes are selectively arranged in the data field of the same slot. The data constructed into a slot are transformed to preselected symbols by a modulating section using a .pi./4 shift DQPSK (Differentially Encoded Quadrature Phase Shift Keying) or similar scheme. The symbols are modulated by orthogonal modulation or similar modulation, superposed on a carrier of preselected frequency, and then transmitted.
On receiving a signal from a mobile station, a base station multiplexes it with slots received from other mobile stations at a full rate over up to three channels. Then, the base station transmits the multiplexed slots to the mobile stations as a frame signal.
Each mobile station removes the carrier from the received signal and demodulates the channel assigned thereto by, e.g., orthogonal detection, thereby detecting received baseband symbols. Noise contained in the received symbols due to fading and other causes is cancelled by, e.g., an equalizer. Then, the original slot signal is restored by differential logic decoding. The restored slots each including the speech data or the control signals are decomposed into independent signals. These signals are each output to either a control section or voice decoding section. As a result, the reproduction of the speech data, position control and other functions assigned to the mobile station are executed.
On the other hand, in the North American CDMA digital cellular system, a signal lying in the speech band is encoded to speech data by, e.g., variable rate speech encoding called QCELP (Qualcomm Codebook Excited Linear Prediction). This speech coding scheme changes the transmission rate in accordance with the ratio of the duration of a speech. For example, for a 20 millisecond frame format, speech data are encoded at code rates of sixteen bits (0.8 kilobits per second or kbps), forty bits (2.0 kbps), eighty bits (4.0 kbps) and 172 bits (8.6 kbps). CRC codes are added only to the speech data encoded at the rates of 8.6 kbps and 4.0 kbps, thereby producing a 9.2 kbps code and a 4.4 kbps code. Further, tail bits are added to all the encoded speech data in order to cause them to converge to the same condition when decoded. As a result, there are produced 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps codes. The signals with the tail bits are each constructed into a symbol by convolutional coding and then caused to recur in accordance with the respective code rate. Consequently, the symbols are provided with a uniform symbol rate, e.g., 19.2 kbps. Further, these signals are subjected to interleaving in order to form respective frames. The frames are each subjected to spread coding by false noise and then subjected to orthogonal transformation using, e.g., a Walsh function. The transformed signals are divided into two phases and subjected to spectrum spreading by OQPSK (Offset Quadrature Phase Shift Keying) modulation using a short PN code or similar pilot false random number sequence. The resulting signals are individually superposed on a carrier and then transmitted.
A base station having received a signal from a mobile station multiplexes it with signals received from other mobile stations in the same frequency band over up to fifty-five channels.
On receiving the OQPSK signal from the base station, the mobile station demodulates it to output a baseband signal. The demodulated baseband signal is reconstructed into the original signals by, e.g., a rake receiver circuit called a finger circuit and effecting synchronization to the pilot false random number sequence, frequency synchronization, and inverse spreading. The reconstructed symbols are deinterleaved and then decoded by Viterbi decoding. At this instant, the signals each having a particular rate are re-encoded and then compared with the original received signal for a decision purpose. The signal coincident with the original received signal is determined to have the rate of the signal. This signal is subjected to error correction and transformed to the original speech data by a voice decoder or vocoder.
The conventional systems described above have some issues yet to be solved, as follows. In the North American TDMA digital cellular system, information for distinguishing the speech data and the control signals or FACCH signal is not added. This requires the receiver side to determine whether a received signal is speech data or whether it contains FACCH. Also, in the North American CDMA digital cellular system using a variable rate vocoder, information for identifying the code rate is not sent to the receiver side. The receiver side therefore must identify the rate of a received signal.
In any case, the receiver side initially executes channel decoding (error correction) corresponding to the encoder of the channel with all the possible signals. Each decoder calculates the reliability of the respective result of decoding, i.e., estimates channel conditions. Among a plurality of decoders included in the receiver side, the decoders not matching the encoder used at the transmitter side cannot decode the signal correctly. Indexes representative of channel conditions and output from the channel condition estimator of each decoder are expected to have a distribution differing from the decoder matching the encoder to the decoder not matching it. The receiver estimates the signal encoded at the transmitter side by using the above difference.
Under these circumstances, signal decision executing the above identification is significant with the receiver. Incorrect signal decision has a critical influence on the performance of the entire communication system. For accurate signal decision, an implementation for estimating channel conditions which are the key to the decision is essential. For example, a CRC code or similar error correcting code or a known code sequence shared by the transmitter and receiver constitutes an overhead against information to be interchanged. For example, in the North American CDMA digital cellular system, as for the 8.6 kbps and 4.0 kbps codes, a twelve-bit and an eight-bit CRC error detecting code are added to an eighty-bit information source code. These two error detecting bits can be used to estimate channel conditions. However, the 2.0 kbps and 0.8 kbps codes are not provided with any error detecting code. Moreover, because channel condition estimation using any of the above schemes includes the specifications of an encoder, it cannot be added to a communication system whose specifications have already been established.
Technologies relating to the present invention are disclosed in the following documents:
(1) "TIA/EIA/IS-54B Cellular System Mobile Station--Base Station Compatibility Standard", section 2.4.5.4.1.1 Bit Error Rate (BER) Measurement Technique; PA1 (2) "TIA/EIA/IS-95 Mobile Station--Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System", section 7.1.3.5.2 Forward Traffic Channel Structure; PA1 (3) Hideki Imai "Code Theory", the Institute of Electronics, Information and Communication Engineers of Japan, section 5.3 Error Detection by Cyclic Code; PA1 (4) "Q0256 K=7 Multi-code rate Viterbi decoder Technical data sheet", FIG. 9, "Re-encode and compare circuit"; and PA1 (5) "A Viterbi algorithm with soft-decision outputs and its applications", J. Hagenauer, P. Hoeher, GLOBECOM-89, pp. 1680-1686.